The difference between the two is that Netflix has a dedicated userbase, who are willing to jump through hoops to get their service; it's the only reason I've installed Silverlight, for example. Youtube, on the other handbasically everyone has watched a Youtube video at one point or another, even if they didn't visit the Youtube site, thanks to embedded videos and such. If I'm a typical user, and Netflix stops working on Firefox, I'm probably going to blame Netflix. If embedded videos stop working in Firefox, I'm much more likely to blame Firefox.
Just a theory, anyway.